The present invention generally relates to a method for destruction of metallic carbon nanotubes, a method for production of aggregate of semiconducting carbon nanotubes, a method for production of thin film of semiconducting carbon nanotubes, a method for destruction of semiconducting carbon nanotubes, a method for production of aggregate of metallic carbon nanotubes, a method for production of thin film of metallic carbon nanotubes, a method for production of electronic device, a method for production of aggregate of carbon nanotubes, a method for selective reaction of metallic carbon nanotubes, and a method for selective reaction of semiconducting carbon nanotubes. More specifically, the present invention, for example, can be applied to thin-film transistors (TFT) in which thin film of carbon nanotubes is used as the channel material.
Among the promising semiconductor electron materials of next generation is semiconducting single-wall carbon nanotubes. This is because semiconducting single-wall carbon nanotubes exhibit not only better electrical properties than silicon (as the major channel material of TFT) but also outstanding mechanical properties that will permit their application to flexible electronics. A field effect transistor (FET) that operates at room temperature with one single-wall carbon nanotube was realized in 1998 for the first time. See, Tans, S. J. et al., Nature, 1998, 393, 49. The possibility of inverter (as one of the simplest logic gates) has been proven with unipolar and complementary carbon nanotube FETs. Other logical gates such as NOR, AND, and SRAM have been constructed in the complementary or multistage complementary mode. A ring oscillator with a maximum oscillating frequency of 220 Hz was formed from a simple array of p-type and n-type carbon nanotube FETs. See, Bachtol, A. et al., Science, 2001, 294, 1317; Derycke et al., Nano Lett. 2001, 1, 453; Javey, A., Nano Lett. 2002, 2, 929. A 2.6 GHz transistor of carbon nanotubes has recently been demonstrated. See, Li S. et al., Nano Lett. 2004, 4, 753.
Attempts to apply single-wall carbon nanotubes to semiconductor electronics have been unsuccessful so far because they contain both metallic ones and semiconducting ones so long as they are synthesized by any exiting technology. Single-wall carbon nanotubes can be metallic or semiconducting depending on their chirality, which is an angle at which the graphite lattice (or graphene sheet) helically rounds about the tubular contour of the nanotube. Metallic carbon nanotubes (which account for about one-third of total nanotubes) greatly aggravate the FET characteristics, such as on/off ratio. It is impossible to adjust the on/off ratio with a network film of untreated carbon nanotubes. In fact, a network film of carbon nanotubes has an on/off ratio lower than 10, which is too small for any practical application. See, Tans, S. J. et al., Nature, 1998, 393, 49. How to address the problem with metallic carbon nanotubes has been a major point in this field.
There have been reported several ways of addressing the problem with metallic carbon nanotubes. The first one is by growing semiconducting carbon nanotubes preferentially. The second one is by separating semiconducting carbon nanotubes from a mixture of metallic carbon nanotubes and semiconducting carbon nanotubes. The third one is by selectively destructing metallic carbon nanotubes in a mixture of metallic carbon nanotubes and semiconducting carbon nanotubes.
There is only one report about the first method as far as the present inventors know. See, Japanese Patent Laid-open No. 2005-45188. This report says that plasma-enhanced chemical vapor deposition (PECVD) at 600° C. yields metallic carbon nanotubes and semiconducting carbon nanotubes in a ratio of 100 to about 89. This result, however, has not yet been confirmed by other researchers. Some researchers suggest that the foregoing result is due to destruction of metallic carbon nanotubes induced by hydrogen plasma rather than preferential growth. See, Hassanien A, NANOTECHNOLOGY 16 (2): 278-281 FEB 2005. Even though preferential grow is possible, there is ample room for improvement in selectivity.
The second method mentioned above has been reported several times in the past three years. The reported methods for separation include alternating current dielectrophoresis (see, Krupke R. et al., Science, 2003, 301, 344), selective precipitation of metallic carbon nanotubes with the help of physical adsorption by octadecylamine or bromine (see, Chattophadhyay D. et al., J. Am. Chem. Soc. 2003, 125, 370; Chen Z. et al., Nano Lett. 2003, 3, 1245), and chromatography of DNA-coated carbon nanotubes (see, Zheng M. et al., Nature materials, 2003, 2, 338). The results reported in these papers are unsatisfactory (quantitatively and qualitatively). In addition, the above-mentioned methods are all subject to chemical contamination because of complex physical or chemical processes involved.
The third method mentioned above involves destruction of metallic carbon nanotubes by electric current. (See, Collin P. et al., Science, 2001, 292, 706) This method is based on the fact that semiconducting carbon nanotubes can be made “off” upon application of gate voltage. It is possible to burn out metallic carbon nanotubes by application of a high source-drain voltage in the presence of oxygen. The first disadvantage of this method is that current flowing through metallic carbon nanotubes generates Joule heat that might burn out their adjacent semiconductor carbon nanotubes. The second disadvantage is inefficiency for a device composed of a large number of FETs because each FET has to be supplied with current. There is another known method of depositing carbon nanotubes on a substrate by in-situ CVD or solution process. (See, Chio et al., Nano Lett. 2003, 3, 157; Lay et al., Nano Lett. 2004, 4, 603.)